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Thread: Real LDE.

  1. #1

    Real LDE.

    (Length Disassembler Engine).

    Determining the length of instructions the processor means. For example the opcode 0xFF0F(0F FF) - has ModR/M byte:
    0F FF 05 DISP32 - 7 byte's.,60.15.html

  2. #2
    Whats the "real" part in the name for?
    I saw some years ago, z0mbies in particular..i think theres a decent one with beaengine too. also for x64... they are quite useful things, wrote my own years ago and discovered how crazy intels encoding is, with exceptions to the 'rules' etc... Fun times

  3. #3
    Quote Originally Posted by evlncrn8 View Post
    Fun times
    Pros start having fun when normal people start getting pissed off. It's the attitude that makes a great reverser.
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  4. #4
    It determines the actual length, even for non-existent instructions. No tables, run the instruction on a page boundary. Even the privileged.

    Other engines use a opcode tables. This makes it impossible to determine the lengths of instructions that are in the tables are not described. For the kernel this engine is unnecessary, there should just handle faults, without adjustment Ss/Esp

    To write to a different platform(eg. x64), no need composes new tables.

    As example, determine(bea, z0mbie etc) the size 0F FF 05 =)
    Last edited by Indy; October 16th, 2011 at 00:31.

  5. #5
    Hmm interesting, will have to check it out when i have a bit of time (probably monday).. totally agree about the tables etc... Damn wish i had time now, your post really got me curious now indy :-)

  6. #6
    epic fail не ?

  7. #7
    Hmm am i wrong or is it using tracing and seh to obtain the : executing the line (with some intelligence in its checking).. ? kinda dangerous if the opcode is unknown but can be

    Not sure what you meant by fail though...

  8. #8
    The code is not dangerous, opcode does not matter. Code is reduced to the correction because of bugs in the kernel.

    The problem with seh is only one - NX. Solved as follows:

  9. #9
    my meaning for dangerous was simply that executing 'unknown' code can be dangerous (eg: sigt instruction that can run in ring 3 with a little help from a ring 0 driver) etc...

    its two approaches. passive (tables) or non passive (tracing) etc....
    im just a pacifist heh

  10. #10
    sidt fword ptr ss:[esp + eax + 0x100]
    0F 01 8C 04 00 01 00 00, returns 8
    sgdt fword ptr ds:[edi + esi - 1]
    0F 01 44 37 FF, returns 5.

    Tracing is used to control the successful execution of instructions.

    executing 'unknown' code can be dangerous
    Really ?
    	mov Seed,2
    	lea edi,Buffer
    	lea ebx,[esi + MAX_INSTRUCTION_LENGTH]
    		invoke RtlRandom, addr Seed
    		mov eax,Seed
    	.until Edi >= Ebx
    	inc Iters
    	invoke LDE, RegionBase, Esi
    Give an example of dangerous code.
    Last edited by Indy; October 18th, 2011 at 23:22.

  11. #11
    0F 50 E0 movmskps esp,xmm0 - dangerous instruction.

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